Cardiac defibrillators and pacemakers are commonly designed to be implanted within a human patient. Such cardiac defibrillators include an electrical energy storage component as part of a power supply designed to provide repeated burst discharges of several joules of electrical energy. Cardiac pacemakers include similar storage components designed to supply lower energy bursts but much more frequently. Both devices therefore require energy storage components of large capacity in order to reduce the number of occasions on which the device must be explanted to renew its energy storage component. It is therefore advantageous that the energy storage component be both compact and capable of large energy storage. It is also advantageous if the energy storage component can be configured to the shape of the overall device, which is typically a flat, disc-shaped configuration to facilitate implantation subcutaneously in the patient. It is well known that aluminum electrolytic capacitors have some properties that are suited for this purpose.
A capacitor of this type conventionally includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed Kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. Typically, the electrolytic or ion-producing component of the electrolyte is a salt that is dissolved in the solvent. The electrolyte thus provides ionic electrical conductivity from the cathode to an oxide layer that is typically formed on the aluminum anode and that functions as a dielectric layer between the anode and the cathode.
Conventionally, the entire laminate is rolled up into the form of a substantially cylindrical body that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered structure of electrode materials with separators interposed therebetween.
Conventional capacitors that employ a liquid electrolyte are subject to leakage, which can damage electrical components and lead to failure of the device. Sealing the device hermetically is not an adequate solution of this problem because of gases that may build up within the device. Expansion chambers adapted to receive the gases have been provided to deal with such problems, but that has led to the disadvantage of even a larger size of the capacitor. Moreover, a liquid electrolyte commonly causes the aluminum oxide dielectric layer on the aluminum anode to de-form, and although the potential across the electrodes can result in currents that re-form the oxide layer, the de-formation results in a shorter lifetime of the formed oxide layer.
U.S. Pat. No. 4,942,501 and its continuations, U.S. Pat. Nos. 5,146,391 and 5,153,820, suggested overcoming these problems by replacing the liquid electrolyte; they provided an electrolytic capacitor that instead employed, between its anode and cathode, a layer of solid electrolyte comprising a solid solution of a metal salt in a polymer matrix, thereby completely eliminating the need for a mechanical separator. These capacitors are immune to leakage and are smaller than prior electrolytic capacitors of comparable construction and operating properties. The preferred method of constructing these capacitors is to deposit onto the surface of the anode a liquid prepolymer electrolyte mixture containing the salt, and then to cause polymerization to take place to cure the electrolyte. The cathode is thereafter formed by deposition upon the surface of the cured electrolyte layer. Similar solid electrolytes are disclosed in Japanese Patent Application No. JP 4-184811, although it is suggested therein that the electrolytes be integrated with a mechanical separator, such as Kraft paper or a porous film or a fabric, so as to increase the mechanical and physical strength of the solid electrolyte.
As noted, these electrolytes differ from conventional aluminum electrolytic capacitor electrolytes in that they are solids; they can exhibit a range of elastomeric material properties ranging from low elastic modulus and high elongation at break, to high elastic modulus and relatively low elongation at break, depending upon the extent to which the polymer is crosslinked. However, in these prior art crosslinked solid polymer electrolytes, limited solubility of the salt in the liquid prepolymer,electrolyte mixture is a common problem.
Most salts exhibit their maximum conductivity at a concentration in excess of 1 mole of salt per kg of solvents (the solvents are typically a mixture of a polymerizable monomer and a plasticizer); however, in some cases the salt of interest is not soluble even to this extent in the desired solvent combination over the desired temperature range. The best achievable prior art solid polymer electrolytes therefore have a conductivity which is less than optimum in some cases. The ESR of capacitors incorporating such electrolytes is undesirably increased as a result. Moreover, this problem becomes even more acute at elevated temperatures (i.e., temperatures in the region 60.degree.-110.degree. C.), since it is known that at such temperatures the solubility of a salt in such a polymer/plasticizer mixture drops below that pertaining at room temperature, as is the case with most non-aqueous solutions. Thus, environmental temperature cycles can result in salt precipitation, which is only very slowly reversed at room temperature.